1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus equipped with the same.
2. Description of the Related Art
An image forming apparatus which adopts an electrophotographic method is equipped with a high voltage power supply apparatus (hereinafter also referred to as a high voltage power supply) adapted to generate a high voltage, and the high voltage power supply is indispensable for an image forming process with respect to recording material. The high voltage power supply apparatus can be any of various modularized power supplies depending on application, including, for example, a power supply for charging, power supply for developing, power supply for transferring, and power supply for fixing. The modularized high voltage power supplies have different specifications according to the configuration of the image forming apparatus. For example, a superposed AC-DC high voltage power supply configured to superpose an alternate current voltage with a direct current voltage is used as a power supply for charging. The superposed AC-DC high voltage power supply controls an alternate current voltage value and direct current voltage value according to load conditions (e.g., load capacity of a charge roller) in order to form images properly in an image forming possess.
In a typical configuration, the superposed AC-DC high voltage power supply is equipped with an alternate current voltage generating transformer and direct current voltage generating transformer and adapted to generate an alternate current voltage superposed with a direct current voltage, using the two transformers. Inexpensive space-saving configurations of high voltage power supplies include a configuration which is set up by reducing the number of transformers in the above-described configuration and in which a high voltage power supply is made up of one transformer and a transistor (see Japanese Patent Application Laid-Open No. 2009-133997). FIG. 6A is a circuit diagram of a high voltage power supply adapted to output a sine-wave alternate current voltage superposed with a direct current voltage of a negative potential. The high voltage power supply in FIG. 6A includes a transformer 103, driving circuits 104 and 106, an AC current detection circuit 105, a direct current voltage detection circuit 107, a half-wave rectification circuit 150, and a transistor 162. The driving circuit 106, which is a circuit adapted to control driving of the transistor 162, compares detection results produced by the direct current voltage detection circuit 107 with a direct current voltage control signal 102 which indicates a target direct current voltage value and thereby performs control so as to maintain a direct current voltage value generated at an output end 108 at a predetermined voltage value. A current flows through the transistor 162 when a voltage on a high voltage side of a smoothing capacitor 142, i.e., an output voltage of the half-wave rectification circuit 150, is higher than a ground potential (hereinafter referred to as a GND potential). That is, a current flows through the transistor 162 when a peak voltage of an output voltage Vout171 is higher than the GND potential. When the peak voltage of the output voltage Vout171 is higher than the GND potential, since a current flows through the transistor 162, the driving circuit 106 can control the direct current voltage by driving the transistor 162. Note that configuration and operation of the circuitry in FIG. 6A will be described later.
For example, along with downsizing and material changes of a charge roller and the like which act as loads on the high voltage power supply, an amplitude range of the alternate current voltage of the power supply for charging is decreasing. With the conventional superposed AC-DC high voltage power supply shown in FIG. 6A, when the amplitude of the alternate current voltage falls below the absolute value of the direct current voltage, the peak voltage of the output voltage Vout171 becomes lower than the GND potential, and consequently a voltage of a positive potential cannot be produced in the smoothing capacitor 142. As a result, since no current flows through the transistor 162, the driving circuit 106 can no longer control the direct current voltage by driving the transistor 162. This disables the use of the above-described configuration which uses one transformer and a transistor. On the other hand, when a configuration with two transformers is used, the increased number of transformers causes a cost increase as well as an increase in transformer installation space.